G.A. Webster

Residual stress distributions and their influence on fatigue lifetimes

Residual stresses can have a significant influence on the fatigue lives of structural engineering components. For the accurate assessment of fatigue lifetimes a detailed knowledge of the residual stress profile is required. The neutron diffraction non-destructive method of measuring these stresses is described. Examples of the detrimental residual stress distributions introduced by welding and of the beneficial stresses produced by such processes as autofrettage, cold hole expansion and shot peening are presented. It is demonstrated that, provided an accurate knowledge of the residual stress profiles generated is available and allowance is made for stress redistribution and the multiaxial nature of residual stresses, reliable predictions of fatigue performance can be made.

Prediction of Creep Crack Growth from Uniaxial Creep Data

In this paper uniaxial tensile creep data are used in conjunction with fracture mechanics concepts to predict creep crack growth rates in materials having a wide range of creep ductilities. A model is proposed of creep damage accumulation in a process zone ahead of the crack tip. The model allows all stages of creep to be incorporated in an approximate manner and creep ductility to be stress and stress-state sensitive. Good agreement is obtained with experimental crack growth data on a range of low alloy steels, a stainless steel, an aluminium alloy and a nickel-base superalloy. It is found that cracking rate is insensitive to the creep process zone size but inversely proportional to creep ductility. Crack growth rates under plane strain conditions are shown to be about fifty times those for plane stress loading.

Fracture mechanics analysis of a crack in a residual stress field

The standard definition of the J integral leads to a path dependent value in the presence of a residual stress field, and this gives rise to numerical difficulties in numerical modelling of fracture problems when residual stresses are significant. In this work, a path independent J definition for a crack in a residual stress field is obtained. A number of crack geometries containing residual stresses have been analysed using the finite element method and the results demonstrate that the modified J shows good path-independence which is maintained under a combination of residual stress and mechanical loading. It is also shown that the modified J is equivalent to the stress intensity factor, K, under small scale yielding conditions and provides the intensity of the near crack tip stresses under elastic-plastic conditions. The paper also discusses two issues linked to the numerical modelling of residual stress crack problems-the introduction of a residual stress field into a finite element model and the introduction of a crack into a residual stress field.

Energy criteria and cumulative damage during fatigue crack growth

This paper considers the strain energy density generated ahead of a fatigue crack as predicted by elastic and non-linear plastic theory. In both cases this decays inversely with distance ahead of the tip. The process zone just ahead of the tip fails when the accumulated energy density reaches a critical value after a certain number of fatigue cycles. A simple expression for LEFM/EPFM crack growth is derived, demonstrating how appropriate energy values may be obtained from low cycle fatigue (LCF) tests, where damage occurs in the bulk of the specimen. The analysis is extended to additional damage accumulating further ahead of the crack during propagation, i.e. before the arrival of the process zone itself. Predicted crack growth rates are compared with experimental LEFM/EPFM data, mostly at high temperature where more energy data are available.

Determination of residual stress distributions in autofrettaged tubing

Abstract An investigation into the prediction of the residual hoop stress distribution in autofrettaged thick-walled tubing of high-strength low-alloy steel with a diameter ratio of 2·07 is described. Analytical and numerical estimates are compared with experimental measurements made by the Sachs boring and neutron diffraction methods. The significance of the choice of yield criterion, the Bauschinger effect and work-hardening on the predicted residual hoop stress distribution is examined. The results showed that the analysis is sensitive to the particular model of material behaviour assumed and the yield criterion employed. Neglect of the Bauschinger effect causes the residual compression at the bore to be overestimated. Closest agreement with experiment is achieved when the unloading stress-strain behaviour of the material is modelled accurately and the average of the Tresca and Von Mises yield criteria is used.

Relevance of Nonlinear Fracture Mechanics to Creep Cracking

Creep crack growth tests, conducted on contoured double cantilever beam (DCB) specimens are described for aluminium alloy RR58 and a chromium-molybdenum-vanadium steel. The results are analyzed in terms of J, the rate equivalent of the J contour integral, which is a nonlinear fracture mechanics parameter. Direct proportionality is found between crack growth rate, ˙a and j. The treatment is shown to reveal a unification of the linear elastic fracture mechanics and net section or reference stress descriptions of creep cracking.

Comparison of creep crack initiation and growth in four steels tested in HIDA

Abstract In power generation plants and the chemical industries there is a need to assess the significance of defects which may exist in high temperature equipment operating in the creep range. For the life prediction methodology for cracked components developed under the HIDA (High Temperature Defect Assessment) Brite/Euram project, it is necessary to have a verifiable data-base of crack initiation and growth data in order to obtain relevant material properties for use in calculations. This paper examines the methods of analysis used. Four types of steels were tested in the programme. These were P22, 1CMV, 316LN and P91 in the parent, as welded and overaged conditions. The data have been obtained from seven participating laboratories. All the results were analysed in the same way using a programme called zrate developed to follow the ASTM E1457-98 testing standard. The results are compared with other crack initiation and creep crack growth data in the literature and with predictions produced from creep data using a model of the cracking process. It has been found that in all cases the scatter in the data is less than for the generic data in the literature. It has also been found that creep crack initiation and growth data can be correlated most satisfactorily in terms of the creep fracture mechanics parameter C ∗ .

Experimental Investigation of Constraint Effects on Creep Crack Growth

In this work the effects of specimen size and type on creep crack growth rates in stainless steel are examined. Experiments have been carried out on high constraint compact tension specimens (CT) and low constraint centre cracked panels (CCP) of ex-service 316H stainless steel. All testing was carried out at 550°C. Constraint effects have been observed in the data, with the large CT specimens having the fastest crack growth rate and the small CCP specimens the slowest. These trends are consistent with those that would be predicted from two parameter (C*–Q) theories. However, it is found that a constraint dependent creep crack growth model based on ductility exhaustion overpredicts the constraint dependence of the crack growth data.Copyright

Theoretical determination of reference stress for partially penetrating flaws in plates

Abstract The reference stress concept is a very powerful tool for use in the stress analysis of non-linear materials. It can be applied to estimate the deformation and fracture behaviour of flawed and unflawed components. When applied to flawed components it is employed in conjunction with fracture mechanics concepts to predict failure. The aim of this paper is to develop a reference stress solution for partially penetrating defects in plates which are subjected to combined axial and bending loading. A solution relevant to ‘global’ collapse of the entire cross-section containing the defect is obtained. The treatment represents the defect by a circumscribing rectangle. A range of shapes and sizes of defect (i.e. rectangle) is considered. As expected, it is found that smaller reference stress values are obtained for the ‘global analysis’, particularly for large defects, than from those that are produced from estimates based on ‘local’ collapse of the uncracked ligament ahead of the defect.

Weibull stress solutions for 2-D cracks in elastic and elastic-plastic materials

The Weibull stress is widely used as a measure of the probability of cleavage failure. In this work analytical and semi-analytical expressions for the Weibull stress are developed in terms of the remote loading parameters, J or K, and material properties. Results are presented for sharp cracks and notches in elastic and elastic-plastic materials under plane stress and plane strain conditions. The proposed relations enable Weibull stress estimates to be obtained without the need for costly finite element analyses and provide insight into the use of the Weibull stress as a parameter for the prediction of cleavage failure of cracked bodies. The expressions have been verified using finite element techniques and good agreement has been found throughout. The results of the analyses have been used to interpret the mesh size dependence of Weibull stress values obtained from finite element calculations.